Carbon dioxide sensors are becoming increasingly important for monitoring air-quality, measuring metabolic activity of animals and controlling combustion. While there are commercial sensors for air quality monitoring, no reliable sensor exits for high-temperature combustion-related applications. Among several designs for cost effective carbon dioxide (CO2) sensor with high sensitivity and selectivity, the potentiometric type sensor that consists of Na+ or Li+ conductor, a metal carbonate sensing electrode and a reference electrode were thought to be promising because they exhibit satisfactory EMF signals over a wide range gas concentrations and temperatures (see, e.g., Y. Saito and T. Maruyama; Recent Developments of Sensors for Carbon Oxides Using Solid Electrolytes; Mid State Ionics 28 to 30; (1988); pp. 1644 to 1647 and C. Park, C. Lee, S. Akbar, and J. Hwang; The Origin of Oxygen Dependence in a Potentiometric CO2 Sensor with Li-Ion Conducting Electrolytes; Sensors and Actuators B; 88; (2003); pp. 53 to 59). However, such sensors suffer from various drawbacks including but not limited to, limited effectiveness in humid conditions.
Potentiometric sensors based on a Na+ conductor such as NASICON® (see, e.g., Y. Saito and T. Maruyama; Recent Developments of Sensors for Carbon Oxides Using Solid Electrolytes; Mid State Ionics 28 to 30; (1988); pp. 1644 to 1647) attached to a Na2CO3 sensing electrode are reported to respond well to changes in CO2 concentration, following Nernst's equations when dry CO2 is used. The time required for 90% response is, however, reported to be several minutes at 500 to 700° C. A major problem with these sensors is that they suffer from strong interference from humidity.
Attempts to improve the sensor performance by modification of the sensing electrode material are described for the first time in references (see, e.g., N. Miura, S. Yao, Y. Shimizu, N. Yamazoe; High Performance Solid Electrolyte Carbon Dioxide Sensor with a Binary Carbonate Electrode; Sensors and Actuators B; 9; (1992); pp. 165 to 170; and S. Yao, Y. Shimizu, N. Miura, N. Yamazoe; Solid Electrolyte Carbon Dioxide Sensor Using Sodium-Ion Conductor and Li2CO3—BaCO3 Electrode; Japanese Journal of Applied Physics, Part 2,31; (1992); pp. 197 to 199). The sensor using binary carbonate (Na2CO3—BaCO3) reportedly exhibits a faster response than a sensor that employs Na2CO3 alone when BaCO3 content is more than a critical amount (see, e.g., S. Yao, Y. Shimizu, N. Miura, N. Yamazoe; Solid Electrolyte Carbon Dioxide Sensor Using Sodium-Ion Conductor and Li2CO3—BaCO3 Electrode; Japanese Journal of Applied Physics, Part 2,31; (1992); pp. 197 to 199).
Thus, there is a need in the art for improved CO2 sensors that work across a wide range of temperatures, as well as CO2 sensors that are less sensitive to humidity.